Spectacles based on liquid crystal, particularly used for 3D viewing, are known through the document U.S. Pat. No. 6,943,852. These spectacles are composed of a single cell formed by a first and second substrate situated opposite each other. The space between the two substrates forms a cavity accepting a liquid crystal of the nematic or ferro-electric type (without however specifying the configuration for the latter).
For the record, materials based on liquid crystal constitute materials the optical properties of which can be modified, and in particular the birefringence, by applying to them an electrical field created by the application of a voltage between the two substrates. When placed for example between crossed polariser and analyser, a voltage-controlled optical obturator is then obtained. Such an optical obturator based on liquid crystal then observes at least two states according to the application or not of a voltage at the terminals of the two substrates:                a so-called passing state in which it allows the light to pass, and        a closed or blocking state, according to which it does not allow light to pass or according to which it allows only a small part thereof to pass.        
In the aforementioned document U.S. Pat. No. 6,943,852, in order to polarise the input and output light and to pass (or block) the output light, three polarisers are used on each side of the cell:                a first rectilinear polariser of polarisation axis P1 is deposited on the whole of an external face of the first substrate,        a second rectilinear polariser of polarisation axis P1 also is deposited on part of an external face of the second substrate, opposite the vision axis of the right eye, and        a third rectilinear polariser of polarisation axis P2 orthogonal to the polarisation axis P1 is deposited on part of the external face of the second substrate, opposite the vision axis of the left eye.        
The cell of the spectacles is switchable, that is to say the obturation thereof, controlled by voltage, is triggered and timed by a synchronisation signal.
The spectacles of the aforementioned document U.S. Pat. No. 6,943,852 also comprises a switch for activating electronics supplying the cell. Thus the spectacles are constantly supplied provided that the switch is actuated.
Although interesting and effective, this technique known from the document U.S. Pat. No. 6,943,852 has several drawbacks.
A first drawback concerns the number of elements necessary for producing such spectacles. This is because such spectacles necessarily comprise three polarisers, a single cell based on liquid crystal and the electronics generating the synchronisation signal. The manufacturing cost is therefore high.
A second drawback concerns the electrical consumption of this type of spectacles based on nematic liquid crystal. This is because, to be transparent, such a liquid crystal must be supplied continuously. In the document U.S. Pat. No. 6,943,852, adapted electronics enable to control the liquid crystal contained in the cell. These electronics are started up by actuating the switch (“on” position). Conventionally, such spectacles comprise a self-contained battery. Through actuation of the switch, the battery therefore continuously supplies the electronics of the viewing spectacles. There then exists a real risk of forgetting to put the switch to the “off” position and to select faulty spectacles the battery of which is at the end of its life. A change of spectacles is in this case necessary to continue to view the film.
A third drawback concerns the vision perceived by the user as soon as the viewing spectacles are placed facing the field of vision of the user. In “off” mode (switch in the “off” position), that is to say when the cell switching electronics are not activated, the cell is not switched and the vision perceived by the user is a so-called “one-eye” vision. Only one field of vision (right eye or left eye) allows light to pass. This is because the polarisation axis of the single input polariser is crossed with the polarisation axis of the first output polariser and parallel to the polariser axis of the second output polariser. The light therefore passes through the cell only at the zone defined by the crossed polarisers when it is a case for example of a twisted nematic liquid crystal. This structure therefore imposes on the user a “one-eye” vision. The vision device is therefore not transparent for the user when the switch of the switching electronics is not actuated in the “on” position in order to time the switching of the liquid crystals of the cell and thus prevent “one-eye” vision.